In lubrication systems for internal combustion engines and hydraulic systems for the control of mechanisms, it is important to know the condition of the oil that is circulated in that excess particle contamination will not only increase the wear rate of moving parts, but that it will also indicate the impending failure of a moving part, for example a bearing, and the chemical breakdown or other change in the oil will mean that its flow characteristics have changed and that it will in many times no longer be suited for its intended purpose. Also, in fuel lines, particularly for aircraft engines, it is important to know if particle contamination is present which would block small conduits, particularly during carburation and if there is a chemical change in the fuel so that it would not properly burn. Also, there are many other areas where it is important to know the particle content of a fluid and its chemical composition, for example for oil well drilling or in water purification.
Optical systems having been used in the past, because of their accuracy and convenience in monitoring some of the above characteristics for fluids. Particularly, particle contamination will produce a scattering of light passed through a fluid and chemical breakdown or other change will affect the transmissibility of the fluid for light passing substantially straight through it. However, such systems are susceptible to considerable errors during long term operation in that light source intensity may change due to voltage fluctuations or changes in the light producing element itself; also, window discoloration and other contamination will have similar effects on the light that is eventually picked up by the light responsive sensors. These error causing factors will produce false readings for the desired characteristics. There has been no satisfactory way in which to compensate for these errors.
Although reference signals are known, such signals applied to the above systems would not provide true references, because they employ different light source elements and different pickups from those of the sample system so that a light source variation in the sample system would not necessarily be picked up in the reference system. Thus, such an arrangement would only be a partial solution.
Fluid monitoring systems in the past have been quite bulky, and involving many components that will correspondingly increase the likelihood of failure. Also complete failure indicators are not generally provided.